When AMD introduced the Athlon XP on the OPGA (organic pin grid array) substrate they alleviated the problems seen by some Athlon and Duron owners where overzealous heatsink application cracked the processor in two. The move to fiberglass packaging made allowances for some flexing during heatsink installation and also improved processor performance by lowering impedance. The only downside was the rather ugly brown colour of the chips...

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Source: XPFreaks.com